1. Field of the Invention
The present invention relates to a nonresonant knock sensor mounted to an internal combustion engine such that when a knocking vibration occurs in the internal combustion engine, the vibration is converted into a voltage signal by a piezoelectric element clamped internally, and the voltage signal is conducted out externally as an output signal.
2. Description of the Related Art
Known examples of conventional knock sensors include those in which a sensor main body including a piezoelectric element fitted over a tube portion of a base is clamped using a nut.
In such configurations, fastening of the nut is performed using a tool such as a torque wrench fastening head, etc., to control torque so as to apply a predetermined load to the piezoelectric element, and due to irregularities in mounting torque from the nut, irregularities in coefficients of friction and dimensions of an internal thread portion on the nut and an external thread portion cut into the tube portion, etc., preload acting on the piezoelectric element is not stable, and there have been problems such as irregularities in output characteristics and detecting precision arising during knocking detection.
For the purpose of solving problems such as those described above, the present inventors have proposed an invention, described in Japanese Patent Application No. 2004-301484, that is not yet commonly known.
In this configuration, a stopper ring inserted into the tube portion of the base is pressed down while using a pressing fitting to apply an external load axially, a punch is driven from beside the stopper ring with the sensor main body in a pressed state, and a protruding portion that is formed by driving the punch in is fixed by crimping into an annular groove that is formed on the tube portion in advance.
In a knock sensor configured as described above, the stopper ring is fixed into the annular groove of the tube portion by crimping, and the sensor main body is clamped by a flange portion on the base and the stopper ring, but the protruding portion of the plastically-deformed stopper ring may not be sufficiently press-fitted into the annular groove due to axial misalignment of the punch occurring due to irregularities in equipment, for example, thereby giving rise to irregularities in clamping force, making the preload imparted to the piezoelectric element become unstable, and there have been problems such as output characteristics and detecting precision becoming unstable, etc.